In a time of flight mass spectrometer (TOFMS), ions accelerated by an electric field are injected into a flight space where no electric field or magnetic field is present. The ions are separated by their mass to charge ratios according to the time of flight (or “flight time”) until they reach a detector and are detected thereby. Since the difference in the flight time of two ions having different mass to charge ratios is larger as the flight path is longer, it is preferable to design the flight path as long as possible in order to enhance the resolution in the mass to charge ratio of the TOF-MS. In many cases, however, it is difficult to incorporate a long straight path in a TOF-MS due to the limited overall size, so that various measures have been taken to effectively lengthen the flight length.
For example, the TOFMS disclosed in the Japanese Unexamined Patent Publication No. H11-135060 (Patent Document 1) includes a closed, “8” shaped loop orbit, where the ions are guided to fly repeatedly in the “8” shaped orbit many times so that the effective flight length is elongated. However, in general, TOFMSs using any type of loop orbit (including the “8” shaped one) has a problem, as explained below with reference to FIG. 2, which shows the schematic construction of a TOFMS having a simple, circular loop orbit instead of the “8” shaped one.
Starting from the ion source 1, the ions are introduced through the gate electrode 4 into the flight space 2 and then guided into the circular loop orbit 3 formed within the flight space 2. It should be noted that FIG. 2 omits the electrodes that generate electric fields for keeping the ions flying in the loop orbit 3. After flying in the loop orbit 3 once or a repeated number of times, the ions leave the loop orbit 3 immediately after they pass through the gate electrode 4. Then, they exit the flight space 2 and reach the detector 5 outside the flight space 2. In this process, the flight distance of the ions increases as the number of turns of the ions in the loop orbit 3 becomes larger, and the increase in the flight distance produces a larger difference between the flight times of two ions having close mass to charge ratios and thereby facilitates the separation of the two ions. One problem for this process is that an ion having a smaller mass to charge ratio will fly in the loop orbit 3 at a higher speed and can catch up with another ion having a larger mass to charge ratio while flying in the loop orbit 3 several times. If this happens, the two kinds of ions will simultaneously leave the loop orbit 3 and reach the detector 5 at approximately the same time.
In summary, the above-described type of TOFMS can effectively separate ions having close mass to charge ratios but may face difficulty in separating ions whose mass to charge ratios differ from each other so that an ion having a small mass to charge ratio can catch up with or lap another ion having a larger mass to charge ratio during their flight. This problem is not unique to the construction in which the ions repeatedly fly in a loop orbit in one direction. For example, the same problem can also occur in the case where the ions are made to reciprocally fly in a straight or curved path so as to achieve a long flight distance by increasing the number of reciprocating motions of the ions.
To avoid the above-described problem, the present inventor has proposed a new method in the Japanese Unexamined Patent Publication No. 2006-12747 (Patent Document 2). According to the method, the flight time of an ion having a specific mass to charge ratio is measured either on the injection path along which the ions that have left the ion source 1 travel until they enter the loop orbit 3, or on the ejection path along which the ions that have left the loop orbit 3 after making a predetermined number of turns in the loop orbit 3 travel until they reach the detector 5, under two conditions differing in the effective flight distance of the path concerned. Since the difference in the flight time between the two measurements depends on the mass to charge ratio, it is possible to calculate the mass to charge ratio from the flight time difference. Patent Document 2 also states that, instead of varying the effective flight distance, it is also possible to vary the state of a certain field (e.g. an electric field) that applies a certain force on the ion flying through a predetermined section of the injection or ejection path. This method changes the time required for the ion having a specific mass to charge ratio to pass through the field, thereby causing a difference in the flight time of the ion.
In these methods, approximate mass to charge ratios can be calculated from the flight time difference. These approximate values can be used to distinguish the peaks resulting from plural ions having different mass to charge ratios, determine the numbers of turns, and calculate the exact mass to charge ratios, even if an ion has caught up with or lapped another ion while flying in the loop orbit.
In the above-described methods, a flight time spectrum with the flight time as the abscissa and the signal strength as the ordinate is created for each of the two measurement conditions established by changing the flight distance or the force acting on the ions, and the resulting two spectrums are compared with each other to determine which peak in one spectrum corresponds to which peak in the other. However, if the sample to be analyzed contains many components, the spectrums will have a number of peaks and it will be difficult to determine the correspondence of the peaks. The lack of information about the peak correspondence makes it impossible to calculate the flight time difference and determine the number of turns of each ion corresponding to each peak. Thus, it will be impossible to calculate the exact mass to charge ratios.
To solve the problems described thus far, the present invention provides a time of flight mass spectrometer having a loop orbit or a similar path, which is capable of accurately determining the number of turns of each ion that forms a peak in the flight time spectrum and exactly determining the mass to charge ratio of each ion even if the sample to be analyzed contains many components.